Aerial photographs of the West blast site taken before and after the explosion clearly show the resulting path of destruction. (Layne Smith/Staff Artist)

Deconstructing the blast

Experts say scene yields clues on source, pattern of destruction

Written by:RANDY LEE LOFTIS | ENVIRONMENTAL WRITER

Published May 1, 2013

C hemistry, engineering, physics and fate dictated life and death during milliseconds on a quiet Central Texas night.

Although agencies investigating the deadly explosion at the West Fertilizer Co. on April 17 have said little, experts not involved in the inquiry told The Dallas Morning News that the visual record already yields many clues about what happened, why some buildings survived and others didn't, and why some people lived and others died.

Among them: Random inches and the strength of an apartment wall might have saved people. The building housing the fertilizer apparently did not burn. A school roof apparently lifted up and then crashed.

And the explosion's fuel was, in all likelihood, many tons of ammonium nitrate. The crater and the pattern of destruction say so.

"Look at the damage done," said Richard John Smythe, a chemist with 40 years of explosion investigation experience in Canada and the U.S. "The aerial photos look like there's a 'ring structure' where the blast wave went out.

"That's characteristic and typical of an ammonium nitrate detonation."

Investigators at the scene must assemble a puzzle with many of the pieces mangled, burned and flung over 1,000 feet - chunks of concrete the size of sofas and microscopic bits of chemicals scraped from debris, roadways and even grains of corn.

The state fire marshal's office would not say on Tuesday where or how the fire that preceded the explosion started. Neither would they identify ammonium nitrate as the blast's fuel.

But the widely used fertilizer seems like the logical suspect, experts said. The chemical is in high demand during the current spring planting season.

1. West Fertilizer Co. office: Charred ground suggests it was the fire’s center. Investigators must determine if the fire heated the fertilizer plant to detonation.

2. Fertilizer plant: This building was where solid ammonium nitrate was received, stored and mixed. Ground markings suggest it was not directly involved in the preceding fire. The detonation point is in a north-extending arm.

3. Crater: The size of the crater suggests that ammonium nitrate alone, not the nearby tanks of anhydrous ammonia, were the explosive fuel.

4. Anhydrous ammonia tanks: Two 12,000-gallon tanks of this liquid fertilizer were apparently shielded by a wall. They are mostly intact, indicating no role in the detonation and disproving rumors that firefighters triggered the explosion by spraying water on an ammonia leak.

5. Grain tanks: There is no sign of fire on the tanks, suggesting that grain was not involved in the fire.

6. Rail line: The nearest rail was shoved onto the farthest rail. The only rail car was blown over by the blast. Its contents show that it didn’t start the explosion.

7. Apartments: They were hit at a full 90-degree angle. Second-floor apartments had the worst damage.

8. Nursing home: This facility suffered blast damage but was apparently shielded by the apartments.

9. West Middle School: Two large roof sections were severely damaged, suggesting that negative pressure may have lifted them.

10. Nearest home: This house was 300 feet from the blast site. It was obliterated.

Aerial views

Mysteries remain, such as what caused the preceding fire, whether the fire itself or the heat it generated triggered the detonation, and whether some intermediary chemical contributed.

The official answers will take months or longer to emerge, but aerial views of the visible damage already suggest what they might be. Experts in explosives, blast structural damage and criminal investigation said much can be learned already.

First, a great deal of explosive material went off.

A blast leaves a fingerprint - a crater - that says something about the violence of the explosion. At West, officials said the crater is about 93 feet in diameter. It ripped through three feet of concrete floor slab and then tore apart 10 additional feet of earth.

That's a sign of extreme force, said Keith Holsapple, a professor of aeronautics and astronautics at the University of Washington who studies blast and impact craters.

If the blasting material had been ANFO - the explosive ammonium nitrate and fuel oil mixture used across the world by industries and terrorists - carving such a crater through the equivalent of hard rock would have required a sphere on the surface weighing 800 tons, Holsapple said.

"The exact size would depend on the details of how it was distributed and the exact strength of the concrete floor and underlying soil," he said. "But it does appear that many hundred tons would be required."

While visual evidence is not conclusive, it appears that the fire did not consume the building the company called the fertilizer plant, where dry fertilizer such as ammonium nitrate was received, stored and mixed. If true, that might leave heat from the fire as the trigger.

The crater is where the fertilizer plant's northern portion used to be. Its location discounts several early rumors - that a rail car of ammonium nitrate was the first to explode; that tanks of anhydrous ammonia were involved; and that firefighters caused the explosion by spraying water on the ammonia tanks.

Company office

Next to the fertilizer plant, however, is a building that the visual record suggests might have been the center of the fire. West Fertilizer's office was obliterated in the blast, but the ground around its former site appears charred.

Amateur videos taken before the explosion show the column of flame rising at a sharp angle, but they are unclear in pinpointing the fire's origin.

All 12 men who died fighting the fire were within yards of the crater. Some were found between the two buildings.

With enough heat transferred to a building where a large amount of ammonium nitrate is stored, said Smythe, "you'd blow that barn to matchsticks."

Aerial images also seem to exonerate West Fertilizer's twin tanks of anhydrous ammonia. The tanks were not shredded, as they typically would be if unusual circumstances led them to explode.

However, they were affected by the blast, with scorch marks and physical damage visible on close examination. One tank appears to have lost its valve mechanism. Whether either tank leaked anhydrous ammonia, a poisonous gas, as a result of the damage is not known.

The visual evidence also seems to argue against the involvement of another material the company stored. The grain tanks on either side of the fertilizer plant were blown apart, but do not appear burned - suggesting that a grain fire was not the cause.

The off-site damage tells much of the story.

The blast displaced Union Pacific's rail line, which is about 12 feet higher than the surrounding land, the company said. UP replaced 1,000 feet of track.

Such shock waves are often more damaging to objects above ground level, said Brian Haygood, an engineer with System Engineering and Laboratories of Tyler, a forensic investigations consulting firm.

A building's height can increase the speed and pressure of a blast wave, Haygood said.

That trait also appeared at the apartments on North Reagan Street, about 300 feet from the crater. The apartments showed more catastrophic damage to the second-floor units.

They also took a direct perpendicular hit from the blast wave. Impact angle is an important factor.

"Broadside of a building, you get major damage," Haygood said.

The apartments appear to have shielded the West Rest Haven nursing home, which received some blast damage but did not collapse. Yet other damage scattered throughout the neighborhood shows the signs of another blast characteristic: a low-pressure wave that lifts roofs from homes and buildings and then slams them back down.

Sometimes a house's draperies protrude from between a roof and the top of a wall, Haygood said - a result of a lifted roof.

That appears to be the case at West Middle School, where two large roof segments collapsed. It also happened at many homes that don't look damaged from above.

Piecing it together

Although no official has indicated evidence of foul play, the site is being treated as a crime scene. Officials and a spokesman for the company's owners said they are cooperating fully with investigators.

Steve Carman, a retired fire and explosion investigator for the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, said standard procedure is dictating what's happening on the ground.

One ATF team collects witness accounts, videos and photos, Carman said. Another looks for physical evidence. Then the two come together to assemble the story.

"They're looking at things that got moved, burned, shattered," said Carman, now a consultant. "Mostly, they're just going to be trying to see what makes sense."

Carman said his experience tells him that a large fire and a large storehouse of ammonium nitrate would explain what happened at West.